1. Field of the Invention
This invention relates generally to apparatus for securing trays that are used in storing or transporting components such as semiconductors, and more particularly to a clip for securing a tray and cover or stack of trays and a cover that places stack compression forces on tray perimeter rails in order to minimize tray distortion caused by clip forces.
2. Description of the Prior Art
Small components such as semiconductors are often stored or shipped in molded plastic trays such as tray 10 shown in FIG. 1. With the components (not shown) in pockets 12, a cover such as cover 14 is placed over the tray 10 for captivating the components in the pockets 12. It is then often necessary to apply a device to secure the top 14 to the tray 10. For example, FIG. 2 illustrates a two part clip apparatus 16 used to clamp a tray and cover together, or a stack of trays and a cover together. The height of leg 18 is designed to accommodate the height of the stack. FIG. 3 illustrates the use of a two part clip apparatus 20 for securing a cover 22 to a tray 24. FIG. 4 illustrates another type of tray clip 26.
In order for the stack of trays to be secure, the clip provides a compressive force to the stack. Prior art designs apply force to a central area of the bottom of the stack, which eventually causes permanent warp/distortion of the trays. Prongs such as 28 (FIG. 2) are used at the stack top, with the opposing force in the central area of the bottom of the stack. In FIG. 2, each of the extensions 30 slide under the corresponding mating part and provide an interference, causing the base 32 to bend in a manner that results in the clip applying a spring force when the clips are installed on a stack.
FIG. 4 shows another clip apparatus 26 with two leaf springs 34 for application of force to a stack inserted in the clip 26. The designs of FIGS. 2-4 apply a leverage to the tray, operating between each of the top prong contact points and the corresponding area of contact on the central area of the bottom of the stack. The leverage applied to the tray by the spring force is proportional to the distances roughly indicated for example by dimensions D1 and D2 resulting in application of a bending force to the trays. This force causes damage due to the application of the force in the thin and weak central area. As the trays warp over time, gaps are produced between trays in a stack. In some cases, the components stored in a tray can fall through the gaps, or be pinched in a gap, causing damage to the component. In addition, when a stack of trays is removed from a clip, and a tray is presented to an automated pick and place machine, the warped, distorted tray can cause pick-up errors. The problem of warped trays is augmented in situations when trays are recycled i.e. re-used after having been used in a stack and become warped. For example, if a used warped tray is placed in a stack with a new tray or new cover, significant gaps are possible, causing tray components to fall out.